# Is space really expanding?

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Viopia
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How can expanding space be tested in the laboratory?
Hi. I have watched some YouTube videos which suggest space (itself) is expanding. This sounds the same as the distance (itself) between moving objects is increasing. What's the difference? How can this even be proven in a laboratory? It doesn't sound as though it can be measured with a tape measure.

Does this have something to do with 'distance stretches light waves as space expands? Is there any evidence for this as well? I would have thought that stretching light waves out would reduce the amplitude of the light. I have seen an image of an object surrounded by non concentric rings to illustrate this. If this image is correct wouldn't the light waves have so little amplitude that they would be invisible because the number of light waves always stays the same?

If Einstein's special relativity reduces the number of photons per unit of time at the Earth's frame of reference from a type 1A supernova 10.5 billions of light years away why does space also have to stretch in some way? The foreshortening only applies to the Earth's fame of reference.

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Summary:: How can expanding space be tested in the laboratory?
As far as cosmology is concerned the laboratory is the large-scale universe. It's not a branch of physics where we have the option to do significant experiments in the lab. The vast majority of the data is what we pick up through telescopes and, more recently, gravitational wave detectors.

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I would have thought that stretching light waves out would reduce the amplitude of the light. I have seen an image of an object surrounded by non concentric rings to illustrate this. If this image is correct wouldn't the light waves have so little amplitude that they would be invisible because the number of light waves always stays the same? If Einstein's special relativity reduces the number of photons per unit of time at the Earth's frame of reference from a type 1A supernova 10.5 billions of light years away why does space also have to stretch in some way? The foreshortening only applies to the Earth's fame of reference.
I've tried to make sense of this, but I suspect there are too many misconceptions buried in there. I guess you are talking about gravitational time dilation? Not surprisingly, professional theoretical and experimental physicists do take a factor like that into account. It's not something they would just forget about!

Viopia
I've tried to make sense of this, but I suspect there are too many misconceptions buried in there. I guess you are talking about gravitational time dilation? Not surprisingly, professional theoretical and experimental physicists do take a factor like that into account. It's not something they would just forget about!
Thanks for yor reply. I was talking specifically about Einstiens special theory of relativity ie: an explanation of how speed affects mass, time and space. I am particularly interesteted in how the notion of space (itself) expanding can be proven not to be merely the distance between objects increasing as those objects move apart. In other words is the Universe expansion due only to the rate at which objects move apart, or is there something else going on.

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"Is space really expanding" depends on what you mean by "really" and what you mean by "space". The way space is usually defined in cosmology is a surface of constant time-since-the-Big-Bang as measured by people who see the CMB as isotropic. In those terms, none of the observers (galaxies drift a bit, but we are near enough one of those observers, as is every other galaxy we can see) is moving but the distance between them is growing. That's why we say space is expanding.

You can change your definition of space if you like, and then other galaxies can be "just moving away". But it turns out to be a bit like deciding to measure speeds on Earth relative to my car. It's not wrong, and sometimes I personally might do it, but as a global thing it's harder to justify - why my car and not someone else's? And why not just use the customary definition?

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Thanks for yor reply. I was talking specifically about Einstiens special theory of relativity ie: an explanation of how speed affects mass, time and space.
I would say it doesn't. In fact, the first postulate of SR is that there is no concept of absolute motion and hence spacetime is the same (in some sense) for all inertial reference frames.

Likewise, speed doesn't affect mass in any meaninful way, as again there is no concept of absolute motion.
I am particularly interesteted in how the notion of space (itself) expanding can be proven
This is part of the General Theory of Relativity, whereby the overall spacetime of the universe must be expanding or contracting. That's fundamentally, from the theoretical foundations of GR, a prediction that has nothing directly to do with relative motion as understood in SR.
not to be merely the distance between objects increasing as those objects move apart. In other words is the Universe expansion due only to the rate at which objects move apart, or is there something else going on.
So, we have GR predicted an expanding (or contracting universe) and then observations of redshift consistent with this - and not easily explicable otherwise as the relative motion of galaxies. And, given additional observational data, such as the CMBR (Cosmic Microwave Background Radiation), we have theoretical and experimental agreement that the universe is expanding.

And that's where modern cosmology begins, I guess.

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speed affects mass
Please forget you ever heard that. It's dropped almost completely out of use in professional circles because you end up having to have rest mass and at least two different relativistic masses which are actually synonyms for total energy and something else I forget, and it's a confusing mess. Mass is invariant, and relativistic mass was a bad idea that we just need popsci to stop using.
I am particularly interesteted in how the notion of space (itself) expanding can be proven not to be merely the distance between objects increasing as those objects move apart.
I'm not sure "space is expanding" is the best way of describing what's going on, but I can't come up with better (except the actual maths). As PeroK says, though, there's an awful lot of experimental support for the maths, and that's what we actually test.

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"Is space really expanding" depends on what you mean by "really" and what you mean by "space".
And expanding too.

Viopia
If you are interested in learning the history how how this was discovered, try Wikipedia.

https://en.wikipedia.org/wiki/Hubble's_law#Discovery
I have looked at Wikpedia and it says ''Although anythng in a local reference frame cannot accelerate past the speed of light, the limitation does not restrict the expansion of the metric itself''. My question is- : who determines what distance can be considered as ''local'', and why does Einstein's time dilation equation not apply to all distances?

Viopia
Please forget you ever heard that. It's dropped almost completely out of use in professional circles because you end up having to have rest mass and at least two different relativistic masses which are actually synonyms for total energy and something else I forget, and it's a confusing mess. Mass is invariant, and relativistic mass was a bad idea that we just need popsci to stop using.

I'm not sure "space is expanding" is the best way of describing what's going on, but I can't come up with better (except the actual maths). As PeroK says, though, there's an awful lot of experimental support for the maths, and that's what we actually test.
But wouldn't particle accelerators (synchrotrons) not work if the energy of being 'kicked along' at the same frequencies was not increasingly absorbed by relativistic mass rather than increased speed due to special relativity? This enables particles of varying masses to be used in the same synchrotron. It appears that Einstien used the word mass because Newton described mass as the resistance to a force acting upon it. What else could be used in place of mass?

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I have looked at Wikpedia and it says ''Although anythng in a local reference frame cannot accelerate past the speed of light, the limitation does not restrict the expansion of the metric itself''. My question is- : who determines what distance can be considered as ''local'', and why does Einstein's time dilation equation not apply to all distances?
Time dilation applies only in flat spacetime. It's not really a thing generally. Popular science sources make a lot of fuss over time dilation, but it's not something that appears very much apart from the introduction to SR.

The general law that applies to all spacetime is that the time each clock measures is the length of its path through spacetime.

Generally curved spacetime reduces to approximately flat spacetime locally, where local means local enough that the effects of curved spacetime are negligible. It does not apply where the expansion of space is relevant.

For example, local might mean within the particle accelerator at CERN. But, across the Earth's surface is not local.

MidgetDwarf
Before worrying about particle accelerators, or anything involving GR/SR etc. Lay off the pop science, and crack open an introductory physics book. Then move onwards to more advance textbooks. The questions you are asking, even if a person gives you a correct answer, cannot be understood, when ones only source of "physics" is popsci books.

Keith_McClary
benrg at astronomy.stackexchange.com says
There's no difference between relative motion and "space expanding" between objects. There is a very common misconception that there is a difference, which has even made it into textbooks, but there isn't actually a difference.

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But wouldn't particle accelerators (synchrotrons) not work if the energy of being 'kicked along' at the same frequencies was not increasingly absorbed by relativistic mass rather than increased speed due to special relativity?
Particle accelerators will work whether you use confusing and outdated terminology to describe them or not. They increase the energy of the particles and the particles' speeds increase. The speeds increase less than they would if Newtonian physics were correct, but it's not - energy and speed do not have the same relationship in the two theories. Invoking "relativistic mass" in this explanation is a bizarrely complicated way of saying that the Newtonian relation between kinetic energy and speed is only approximate, and not accurate at high speed.
What else could be used in place of mass?
You just say that the kinetic energy of a particle of mass ##m## (note: I do not mean relativistic mass) traveling at speed ##v## is ##(\gamma-1)mc^2##, where ##\gamma=1/\sqrt{1-v^2/c^2}##. This is approximately ##\frac 12mv^2## when ##v## is small compared to ##c##, which is why Newtonian physocs works at low speeds, but means that adding energy results in a lower increase in speed than Newtonian predictions.

Why do we need an explanation structured in terms of Einstein being a correction to Newton's thinking? The reality is just that Newton did not have access to high enough energies to realize that his theory is only an approximation. Trying to shoehorn a more general and powerful theory into the boxes laid out by the less general theory is never going to work well.
He's not wrong. I'd say that he's being a bit strong, because "space expanding" is a very special case of relative motion between objects and it's a useful way of thinking, but he's not wrong.

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You have to be careful, however, not to interpret this as GR is just SR with allowed superluminal velocities.

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Thank you for answering my initial question. It's so good to have a discussion after the Covid lockdowns. I often wonder if, due to the whole universe developing from a Big Bang, if the Universe is 'flat' because time dilation (sorry, foreshortening of distances) makes it imposible for the universe to go beyond a certain distance from our frame of reference here on Earth. In other words there has been insuficient time for the universe to expand beyond a certain point from the Earth's perspective.

Viopia
Particle accelerators will work whether you use confusing and outdated terminology to describe them or not. They increase the energy of the particles and the particles' speeds increase. The speeds increase less than they would if Newtonian physics were correct, but it's not - energy and speed do not have the same relationship in the two theories. Invoking "relativistic mass" in this explanation is a bizarrely complicated way of saying that the Newtonian relation between kinetic energy and speed is only approximate, and not accurate at high speed.

You just say that the kinetic energy of a particle of mass ##m## (note: I do not mean relativistic mass) traveling at speed ##v## is ##(\gamma-1)mc^2##, where ##\gamma=1/\sqrt{1-v^2/c^2}##. This is approximately ##\frac 12mv^2## when ##v## is small compared to ##c##, which is why Newtonian physocs works at low speeds, but means that adding energy results in a lower increase in speed than Newtonian predictions.

Why do we need an explanation structured in terms of Einstein being a correction to Newton's thinking? The reality is just that Newton did not have access to high enough energies to realize that his theory is only an approximation. Trying to shoehorn a more general and powerful theory into the boxes laid out by the less general theory is never going to work well.

He's not wrong. I'd say that he's being a bit strong, because "space expanding" is a very special case of relative motion between objects and it's a useful way of thinking, but he's not wrong.

Viopia
Yes, think 'relativistic time dilation' would be a good alternative to 'relativistic mass'.

ergospherical
To a cloud of dust (or a set of galaxies) is associated a number ##\theta## called the expansion which expresses the fractional rate of change (per time) of the volume they collectively occupy. Expanding space corresponds to non-zero values of ##\theta##. Mathematically ##\theta## is the divergence of the vector field representing the flow of the dust particles (or galaxies) through spacetime.

Viopia
To a cloud of dust (or a set of galaxies) is associated a number ##\theta## called the expansion which expresses the fractional rate of change (per time) of the volume they collectively occupy. Expanding space corresponds to non-zero values of ##\theta##. Mathematically ##\theta## is the divergence of the vector field representing the flow of the dust particles (or galaxies) through spacetime.
Does 'Expanding space corresponds to non-zero values' mean that foreshortening, or time dilation, cannot be expanded to infinity? If so, there is a convergence distance the universe cannot expand beyond from the Earth's frame of reference.

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I often wonder if, due to the whole universe developing from a Big Bang, if the Universe is 'flat' because time dilation (sorry, foreshortening of distances) makes it imposible for the universe to go beyond a certain distance from our frame of reference here on Earth. In other words there has been insuficient time for the universe to expand beyond a certain point from the Earth's perspective.
It would be better to read about inflation and the Big Bang model than try to invent your own terminology. The size of the observable universe is limited by how much time light has had to travel since the Big Bang (or, more accurately, since the universe became transparent to EM radiation).

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Does 'Expanding space corresponds to non-zero values' mean that foreshortening, or time dilation, cannot be expanded to infinity?
As far as that makes any sense, no. There is no upper limit to the size of the universe. In fact, current best model has an infinite universe.
If so, there is a convergence distance the universe cannot expand beyond from the Earth's frame of reference.
No. Even if the universe is finite, there is no upper limit on its size.

Viopia
As far as that makes any sense, no. There is no upper limit to the size of the universe. In fact, current best model has an infinite universe.
You are choosing just one of the many theories about the expansion of the universe which many people seem to have heard about. Some of these theories are really weird. My evidence is the graph I have plotted (see attached photo). I suggest you read the article, which can be found on Google, entitied ''Does observational evidence indicate the universe is expanding? - Part 1: the case for time dilation'' by John Hartnett''. There is some interesting research going on but, if you read this paper in full, especially the last paragraph in the ''conclusions'' you will notice that there is exreme confusion over this issue and it reminds me of, what I call, the ''Jean-Jacques Bachelier effect''. Bachelier used Brownian motion mathematical modeling to value stock options. Up until his PhD thesis ''the theory of speculation'' was discovered mathematicians were in disarray over the mathematical modeling to value stock options. Their models became more and more complicated, containing many symbols which no numbers could be attributed to, because they didn't really understand the problem - until they discovered Bachelier's thesis of course. I believe the same is happening with all the theories about the Universe. I have simplified the problem of determining the distance of type 1A supernovae and my result is that the most distant type 1A supernova is not 10.5 billlion light years away from the light emission, but is only 5 billion light years away
I do get all my information from YouTube videos and by searching the internet and so there is a high probability that I may have misunderstood something, or my calculations may be wrong, but it seems to agree with the ''Boomerang'' results in the conclusion that an accelerating universe is unlikey

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weirdoguy
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You are choosing just one of the many theories about the expansion of the universe which many people seem to have heard about. Some of these theories are really weird. My evidence is the graph I have plotted (see attached photo). I suggest you read the article, which can be found on Google, entitied ''Does observational evidence indicate the universe is expanding? - Part 1: the case for time dilation'' by John Hartnett''.
Sorry, I'm not interested in what a young-Earth creationist has to say. If you're intent on presenting pseudo-science, you've come to the wrong place.

vela
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I do get all my information from YouTube videos and by searching the internet….
Bluntly, if you have been relying on internet searches and YouTube videos for anything but entertainment you have been wasting your time - it is not possible to learn the real science that way.

Please take a moment to look at the forum rules about acceptable sources. This thread is closed as it violates that rule.

weirdoguy, berkeman and PeroK